The present invention relates to communications channels, all of which are inherently limited in their capacity (or rate) of information transfer by channel impairments. More specifically, the present invention relates to compensating reference frequency drift in a communications system between a plurality of Cable Modems (alternatively referred to as “CM”) and a Cable Termination System (alternatively referred to as “CMTS” or “headend”), where the system requires critical upstream timing.
Communication systems are subjected to impairments, generally of a brief or transitory duration. One example of such impairment is often referred to by the generic term “noise.” Noise sometimes emanates for example, from within electrical components themselves, such as amplifiers and even passive resistors. Another example of impairment is referred to as “interference,” which is usually taken to be some unwanted manmade emission, from another communications system such as a radio or from switching circuits in a home or automobile for example. “Distortion” is a yet another example of such impairment, and includes linear distortion in the channel, such as pass-band ripple or non-flat group delay for example, and nonlinear distortion, such as compression in an overdriven amplifier for example. It is contemplated that there are many other types of impairments that may also adversely affect communications in a channel.
Often, such impairments may by dynamic in nature. In many cases, the impairment may be at one level of severity most of the time. In this instance, the communications system may be designed or optimized in some fashion to operate at that specific level of impairment. Occasionally, however, one or more of impairments may become so severe as to preclude the operation of such communications system optimized for the more ordinary level of impairments.
Previously, when a large interference or burst of noise occasionally impinged upon the receiver (a CM for example), such large out-of-the ordinary bursts of received power are simply blanked out. Often, analog processing means are used, almost at, if not right at, the receiver input. This may be done especially to protect CMs or other sensitive receiver front-ends from damage. While this technique may provide some benefit in circumstances where the noise or interference power dwarfs the signal-of-interest power, it does not protect against the many other impairments that have power more on the order of the signal-of-interest power (or even much less). Thus blanking does not, by itself, provide the receiver with a means to improve its overall performance in the presence of the lost information, i.e., the information content concurrent with the large noise burst.
One known technique, a forward error correction technique (alternatively referred to as “FEC”) has been applied, even unknowingly, to solve this problem. FEC techniques incorporate soft-decision decoding, such as is common with convolutional error correction codes and the Viterbi decoding algorithm. In such correction techniques, as the error power in the received signal increases, such increase is passed directly into the decision process.
Such encoding and decoding techniques have been in common practice for years, and are widely applied without thought to temporary fidelity changes in the channel. Fortunately, in the event of a change in the channel fidelity, the soft-decision decoding takes into consideration the larger error power in making signal decisions. However, unfortunately, often with a change in channel conditions, there is duration of multiple symbol intervals (in a digital communications system for example) where the degradation persists. During this time some symbols may be so severely erred that they actually appear close to another possible but incorrect symbol. In such event, the soft-decision decoder actually “thinks” it has received a low error power, and may rate the wrong signal with a high confidence. This becomes much more likely as the constellation density (of a QAM constellation for example) is increased for high rate communications,
Additional techniques, such as a Time Division Multiple Access technique (alternatively referred to as “TDMA”) have been applied to solve this problem. In this technique, one or more carrier frequencies are shared among a plurality of CMs. Known standards, DOCSIS 1.0 and 1.1 for example, each of which are incorporated herein by reference in their entirety, define the physical layer, and additional layers, in which a plurality of CMs transmit data upstream to and receive data downstream from the CMTS or headend. In this technique, each upstream carrier frequency or channel assignment is generally shared by a plurality of CMs, each being granted time slots wherein they may use the channel. These grants are allocated and made known to the CMs via the downstream broadcast transmissions. Some of the grants only enable a single CM to transmit, while other time slot grants are in contention mode. That is some, or all, of the CMs may attempt to use the grant. However, if more than one CM attempts to use a grant in the contention mode, all the CMs will likely be unsuccessful in channel use.
Yet another technique, such as a direct-sequence spread-spectrum modulation technique discussed by J. Young and J. Lehnert, in their paper titled “Analysis of DFT-Based Frequency Excision Algorithms for Direct-Sequence Spread-Spectrum Communications,” IEEE Trans. Comm., vol. 46, pp. 1076–1087, August 1998, the complete subject matter of which is incorporated herein by reference in its entirety, has also been applied to solve this problem. In this technique, frequency excision is used to eliminate narrow-band energy, thus enhancing the capacity of direct-sequence spread-spectrum modulation to reject narrow-band interference. However, this disclosed technique focuses on particular waveforms having energy concentrated about a narrow band.
Yet still another technique, such as a Code-Division Multiple Access technique (alternatively referred to as “CDMA”) discussed by M. Lops, G. Ricci and A. Tulino, in their paper titled “Narrow-Band-Interference Suppression in Multi-user CDMA Systems,” IEEE Trans. Comm., vol. 46, pp. 1163–1175, September 1998, the complete subject matter of which is incorporated herein by reference in its entirety, has also been applied to this problem. In this technique, a decision is made regarding the bit(s) transmitted by each user over a communication system. This decision is based on the projection of the observables on to the orthogonal complement to the subspace spanned by the other users' signatures and the narrow-band interference. The disclosed technique recognizes that the blanking and iterative processing may be performed with an orthogonal basis set decomposition of the frequency domain.
Yet still a further technique, such as a Synchronous Code Division Multiple Access technique (alternatively referred to as “SCDMA”) comprises a spreading technique to transmit symbols at the same time on the same frequency. More specifically, this technique may be used, in one embodiment, with a preliminary DOCSIS 2.0 physical layer standard (alternatively referred to as the “DOCSIS standard”), which is incorporated herein by reference in its entirety. The DOCSIS standard defines the physical layers in which pluralities of CMs transmit data upstream to and receive data downstream from the CMTS or headend.
It is contemplated that in SCDMA, the spreading codes may be cyclical shifts of one 127 chip spreading code, plus one additional chip. Thus, the spreadingIn On codes are nearly cyclical shifts of one another.
For SCDMA modulation to work efficiently, all the spreading codes should be synchronized as they arrive at the receiver (a CM for example). Timing misalignments result in inter code interference (alternatively referred to as “ICI”), which may degrade performance. It is known that aligning upstream transmissions from various CMs in a system to within +/−2 nanoseconds may limit such ICI. To accomplish this alignment, an SCDMA CMTS or headend may transmit downstream clock transmissions or reference timings that the various CMs may lock onto. Locking onto such downstream clock transmissions or reference timings enables the CMs to: (1) synchronize the upstream symbol transmissions with this downstream reference timing; (2) provide a frequency lock for the upstream CMs; (3) provide feedback in the initial ranging; and (4) make subsequent measurements available to the various CMs (from the CMTS), providing for correction of the unique delay (i.e., phase) for each CM, and enabling each CM to fall into the required +/−2 nanosecond alignment. In other words, locking onto the downstream symbol clock provides the CM with the necessary frequency information, and the initial (and relatively infrequent) subsequent timing correction measurements needed to provide the phase information to each CM.
The DOCSIS Standard provides standards for allocation of jitter and stability for the CMTS reference clock and downstream symbol clock to facilitate the CMs locking onto such downstream clock transmissions or reference timings. This Standard also sets forth fidelity requirements for the CM in recovering and “turning around” this downstream symbol clock.
Various impairments, interference, distortion or noise in the channel may degrade signal performance. In one embodiment, special techniques may be employed to limit or mitigate the degradation by compensating for reference frequency drift in a communications system.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with the present invention as set forth in the remainder of the present application with reference to the drawings.